1. Field of the Invention
The invention relates to a method and apparatus for measuring the constituent elements of a liquid conductive material. More particularly, the invention relates to measuring the constituent elements of a melt of molten metal alloy.
2. Description of the Prior Art
The physical properties of metal alloys, such as strength, hardness, toughness and corrosion resistance, depend in part upon the composition of the metal. In particular, these properties depend on the quantities of the constituent alloying elements.
To determine the composition of the alloy, a liquid sample is typically drawn from a melt of the molten alloy, allowed to solidify and then analyzed by chemical or spectrometric means. When the analysis indicates the correct composition, the melt is poured to produce the desired products. Efficient production requires a rapid and accurate analysis because continuing chemical processes in the melt can change the composition between the time the sample is drawn and the time the analysis is received. If the time separation is too great, the analysis may not accurately represent the true composition of the melt.
Attempts have been made to provide accurate "realtime" analyses of molten metal alloys. U.S. Pat. No. 3,659,944 issued May 2, 1972 to M. Bojic discloses an apparatus for the direct and continuous spectrometric measurement of molten metals. U.S. Pat. No. 3,645,628 issued Feb. 29, 1972 to M. Bojic et al. provides an apparatus for direct spectrometric analysis of molten metals which has a light conveying tube with a disposable tubular extension. U.S. Pat. No. 3,672,774 issued June 27, 1972 to Bojic et al. provides an apparatus for direct spectrometric examination of molten metals.
U.S. Pat. No. 3,669,546 issued June 13, 1972 to J. M. Virloget disclosure a special optical transmission apparatus which receives light generated by a spark between an electrode and the surface of a molten metal. The apparatus transmits the light with minimum alteration to a spectrographic means.
The spark emission measurement devices of the prior art, however, have many shortcomings. The spark is an inefficient producer of light emissions because the spark absorbs much of the radiation. As a result, the linear dynamic range of the emissions produced is rather short. A further disadvantage of a spark emission analytical technique is the requirement that the sampled melt surface be free of slag and representative of the bulk material. A critical parameter is the geometry of the melt surface and counter electrode. In particular, the gap length, or distance between the counter electrode and melt surface, should remain, at all times, substantially constant. Dynamic changes in gap length can impair the analytical accuracy and precision of the results. The spark emission procedure also produces matrix effects because the material eroded from the electrodes interferes with the light emissions from the excited sample. Since the emitted light must be conducted to a spectrometer, the light becomes scattered and attenuated by intervening atmospheric molecules and by the transmission mirrors and lenses used in the optical systems. This necessitates close placement of the spectrometer near the molten metal and the use of special, complex optics to conduct the light.
Typically, the prior art devices pump molten metal to a sampling point, or take the sample from near the melt surface where the slag can interfere with the analysis. Such procedures are not conducive to obtaining precise insitu analyses. The spark emission devices also produce residual vapors that can interfere with later samples and analyses, and the devices are unable to provide data on the melt composition at varied locations and depths in the melt to ascertain its homogeneity.
U.S. Pat. No. 3,606,540 issued Sept. 20, 1971 to R. V. Williams, et al. uses a gas stream to atomize a molten metal. The molten droplets in the form of a spray are conducted by a stream of gas through a tube conduit to a plasma spectrometer. The spectrometer analyzes the droplets, and a special gas barrier formed in the conduit is intended to keep the metal droplets away from the tube walls. The atomizer-type device of Williams, et al., however, has not been completely satisfactory. The gas automizer droplets are generally too large for efficient transport by a flowing gas. Droplets can still solidify on the transport tube walls and clog the tube. Also, residue from prior samples can interfere with the analysis of subsequent samples.
U.S. Pat. No. 3,602,595 to R. L. Dahlquist discloses a device which creates an electric arc between an anode of a current source and a material to be sampled. The arc creates an aerosol containing droplets of the material, and the aerosol is carried away by a flow of gas for spectrometric analysis. U.S. Pat. No. 3,685,911 issued Aug. 22, 1972 to R. L. Dahlquist, et al. provides an apparatus for producing a stabilized arc plasma in a capillary tube. Material introduced into the plasma is excited to produce light emissions, and these light emissions are analyzed by spectrometer.
The device taught by Dahlquist, however, has not been able to produce reliable or consistently reproducible analyses of a molten material. The surface of the liquid melt ripples and moves, making it difficult to maintain the spark gap required to consistently produce a sufficient quantity of aerosol particles from the spark for analysis.
Thus, conventional devices, such as those discussed above, have been unable to efficiently provide accurate and reliable real-time analyses of molten metals. Spark devices have been inefficient producers of emissions for spectrometric measurement, and these spark systems have required complicated optics to transmit the light to the spectrometer for analysis. Atomization systems have been susceptible to clogged transport tubes, and new samples have been often contaminated by residue left behind by previous samples. Spark devices, which produce an aerosol for subsequent spectrometric analysis, however, have not been able to reliably produce an aerosol from a molten sample that contains a sufficient concentration of particulates for accurate analysis. The electrical arc sometimes caused movement and displacement of the liquid without providing any analytical signal. The intensities of the analytical signal, when present, would greatly fluctuate over time and these fluctuations would unacceptably decrease the precision of the analysis. In addition, material from the liquid being sampled could contaminate the spark electrode and inhibit further analyses.